I have used Pentaho data-integration(kettle directly as well as an ejb
that integrates kettle jobs into a web service) to interface with a wsdl
based reporting service we host internally and it works well using the
steps that kettle comes with. It also should be fairly straight forward
to perform an http request to download google reports(we do not do this,
but I did suggest it internally). It would be reasonable(almost
trivial) to use the data-integration suite for downloading and staging
report data.
I think managing campaigns using pentaho may require significant work.
You would likely not be using the google java api to make this happen,
unless you feel like creating custom kettle plugins for each operation.